Anatomy and Histology. 109 



is not a clean-cut process, such as we see in our common trees and shrubs, 

 but first appears directly opposite either a primary stereome bundle or a 

 primary canal, as an ingrowth, simulating invagination (plate 32, fig. 4). 

 The absciss layer which effects leaf-fall is similarly clumsy, so to speak. 

 This tissue consists of a quite irregular layer of cork-cells, continuous with 

 the primary phellogen. The outermost cells, those, namely, immediately 

 beneath the base of the leaf, first become suberized. 



Until an advanced age is attained, the inner periderm does not cut 

 deeply. In old stems, 20 to 50 years of age, light-colored layers of cork 

 may be seen penetrating to half the depth of the cortical tissues, but quite 

 irregularly. It is of interest to note here that this cork presents a special 

 practical difficulty in the factory in handling the comminuted shrub after 

 it has passed through the pebble-mill. The bagasse is then, with the 

 exception of this cork, which has been broken up into flakes, separated in 

 water, the rubber and the cork flakes floating and the remainder sinking. 

 Only by means of pressure under water or prolonged soaking may the 

 cork be waterlogged, when it sinks, leaving the clean rubber still floating. 

 These layers of cork are seen in plate 2, fig. B, from a photograph of a 

 stem certainly forty years old. 



SECONDARY STRUCTURE. 



The secondary cortex is characterized by alternating concentric rows 

 of stereome bundles and resin-canals. Between succeeding stereome mass 

 and leptome parenchyma (canal-cells, consisting of endothelium and usu- 

 ally a single subjacent or supporting layer), there frequently intervenes 

 no tissue at all, and the stereome occupies the whole of the space between 

 adjacent resin-canals. In the inner part of the secondary cortex one finds 

 alternating canals and "soft" leptome, the composition of which raises 

 some points of question. The canals, as described correctly by Ross 

 (1908), arise as a double row of cells derived directly from the cambium 

 (plate 22, fig. 13). Surrounding the "secreting" cells is at least one layer 

 of leptome parenchyma, the usual condition in slowly growing plants. In 

 irrigated plants there may be two or three (or occasionally more) layers 

 (plate 29, fig. 1). This is followed, radially, by a mass of sieve-tissue 

 (plate 32, fig. 2), which may be regular in transverse outline, and com- 

 pletely uninterrupted by parenchyma until another canal is laid down, 

 or it may be narrow and more or less irregular, as in irrigated plants 

 (plate 25, fig. 8; plate 29, fig. 4). In any event, the sieve-tissue occupies 

 the radially placed space, broadly speaking, between successive canals, 

 and it is in this space that we find stereome later. 



The manner in which the stereome arises is, in broad outline, as 

 follows: The outermost (on the radius) leptome cells undergo transverse 

 enlargement and become stereomatic. Successively other adjacent cells 

 lying farther in behave similarly. The resulting tissue, however, occu- 

 pies more space than did the original cells from which it arose. As the 

 total space which is occupied by the stereome is usually identical with the 

 total leptome, it follows that there must be some readjustment. This is 

 brought about by the discontinuous sclerosis of the leptome, so that irreg- 

 ularly alternating masses of this are destroyed and become compressed. 



